Is beef okay for a cat? All three of my cats love it, but I'm not sure it's good for them.

If they love it and don't have any kind of reaction to it (can be a common allergen), then I see no problem with feeding beef. Bison is another good one to try (although you'll have to modify the "whole prey" thing when feeding stuff like that, obviously), and as a bonus, bison aren't part of the evil factory farming industry like most cattle are.